There are two main types of linear actuators, namely fluid power actuators that operate on differential pressure and electromechanical actuators driven by an electric motor. Electromechanical actuators may be back-driven or self-locking. If an electromechanical actuator is self-locking, then the actuator may not be back-driven by exerting a force on the output. Many aerospace vehicles employ back-driven actuators. The actuators are typically back-driven when a stream of air exerts a force that causes a spoiler that is presently deployed to retract.
The electrical power created by back-driving is conventionally dissipated by elements such as resistors and diodes. In one example, a direct current (DC) motor is back-driven by an H-bridge circuit. An H-bridge circuit is a type of electronic circuit that enables a voltage to be applied across the DC motor in either direction. In the example as described, diodes are used to dissipate the electrical power and to protect other electrical components included in the H-bridge circuit. Specifically, the H-bridge circuit includes transistors that switch rapidly based on a pulse-width modulation (PWM) scheme. The PWM scheme controls either speed or torque of the DC motor. When the transistors are switched off and the supply current to the DC motor is suddenly interrupted, a voltage spike is created since the DC motor is an inductive load. The voltage spike may be referred to as flyback, and the diodes in the H-bridge are used to protect the transistors from the flyback created by the pulsing of the DC motor. The diodes generate heat when dissipating electrical power. Furthermore, the heat created by the diodes rises in response to the pulse rate of the PWM scheme increasing. Moreover, the heat created by the diodes needs to be disposed.
It may be challenging to dispose of the heat generated by the diodes. In fact, heat generation in aerospace application is especially problematic because the components of the vehicle are required to operate within specific temperature ranges during all mission phases. The challenges faced with heat dissipation in an aerospace vehicle may be further compounded since aerospace vehicles also encounter relatively high aerodynamic forces during operation. This results in more work that needs to be performed by an actuator in order to move a surface. Moreover, the actuators are also usually required to operate at very high rates such as, for example, over 100,000 Hertz. Finally, since aerospace vehicles travel at high speeds through space, the air surrounding the space vehicle is already at an elevated temperature. Thus, it may not be possible to release most of the heat generated by the diodes.